How do I fix the UVs on this propane tank?
That is actually an issue with the normals of you mesh, which is the direction the faces are pointing/facing. You can recalculate the normals by selecting all of the mesh in edit mode and pressing Ctrl+N. This could be resulting from double vertices, so you could try removing doubles with W, again in edit mode with all selected. This may affect the exported file (assuming you are exporting it) in an undesirable way, so just be careful and have a backup.